Think of all the progress in science and medicine there's been over the centuries, where we've developed understandings and ways of describing things that we didn't have in the past. The actual things being described always existed, we just didn't understand them or know how to describe them. It's got nothing to do with metaphysics at all. It's about our human limitations.

Speaking from my own experience, I had medical symptoms for over 30 years that I didn't know how to explain. There doesn't seem to be any existing concepts for them.

I have only very recently come to realise that they are probably due to some sort of nerve-related issues -- where this is the reason why they're hard to explain. Something is causing problems with the signals to the brain, so the brain doesn't quite know how to interpret it, which is why you get something indescribable. (or at least that's what seems likely to be going on in my case).